Add boost-regex based envsubst implementation

CMakeLists.txt

@@ -28,6 +28,7 @@ TARGET_SOURCES(
   rvstd
   PRIVATE
     "src/rvstd/boost_json_ext.cpp"
+    "src/rvstd/envsubst.cpp"
     "src/rvstd/unique_identifier.cpp"
     "src/rvstd/tableau.cpp"
     "src/rvstd/options.cpp"

include/rvstd/envsubst.hpp

@@ -0,0 +1,16 @@
+#ifndef RVSTD_ENVSUBST_HPP
+#define RVSTD_ENVSUBST_HPP
+
+#include <boost/regex.hpp>
+
+#include <iostream>
+#include <regex>
+#include <string>
+
+namespace rvstd {
+
+auto envsubst(const std::string& text) -> std::string;
+
+}  // namespace rvstd
+
+#endif  // RYJSOND_ENVSUBST_HPP

src/rvstd/envsubst.cpp

@@ -0,0 +1,35 @@
+#include "rvstd/envsubst.hpp"
+
+#include <boost/regex.hpp>
+
+#include <string>
+
+namespace rvstd {
+
+auto envsubst(const std::string& text) -> std::string
+{
+  static const boost::regex env_var_pattern{R"--(\$(?:(\w+)|\{([^}]+)\}))--"};
+  std::ostringstream result;
+  boost::regex_replace(
+    std::ostreambuf_iterator<char>(result),
+    text.begin(),
+    text.end(),
+    env_var_pattern,
+    [&](const boost::match_results<std::string::const_iterator>& match) {
+      std::string var_name_str;
+      if(match[1].matched) {
+        var_name_str = match[1].str();
+      }
+      else if(match[2].matched) {
+        var_name_str = match[2].str();
+      }
+      if(const char* ptr = std::getenv(var_name_str.c_str()); ptr != nullptr) {
+        return ptr;
+      }
+      return "";  // Empty string for non-existent variables
+    });
+
+  return result.str();  // Return the result stored in the output stream
+}
+
+}  // namespace rvstd